/**
 * 高精度:减法  任意->整数,小数,正负
 * @param a string|number 被减数
 * @param b string|number 减数
 * @return string
 */
$.bcSub = function(a, b){
	let A=(a+='')[0]==='-', B=(b+='')[0]==='-';
	if(A!==B){
		if(B) return $.bcAddDot(a,b.slice(1));
		const rtn = $.bcAddDot(a.slice(1), b);
		return rtn==='0' ? '0' : '-'+rtn;
	}
	if(!A) return $.bcSubDot(a, b);
	return $.bcSubDot(b.slice(1),a.slice(1));
};
